How does SHBG affect testosterone?

Sex Hormone Binding Globulin (SHBG) significantly impacts testosterone by controlling its availability in the body. Most testosterone in the bloodstream is bound to proteins, and about two-thirds of it is specifically attached to SHBG. This binding is so tight that testosterone cannot be easily released and therefore becomes unavailable for cellular uptake. The only form of testosterone that can enter cells is the free form, which constitutes a small percentage of the total testosterone in the blood 1.

The amount of SHBG varies greatly among individuals, and higher levels of SHBG can falsely suggest that a person's testosterone levels are normal, based on total testosterone measurements, when they may actually be low at the cellular level. As a result, focusing on free testosterone is crucial for a more accurate assessment of testosterone deficiency symptoms 1. This insight is especially relevant as SHBG levels tend to increase with age, potentially masking low testosterone symptoms in older individuals 2.